Most modern wind turbines are controlled and regulated continuously with the purpose of ensuring maximum power extraction from the wind under the current wind and weather conditions, while at the same time ensuring that the loads on the different components of the wind turbine are at any time kept within acceptable limits.
During operation the wind turbine tower is seen to move and oscillate. Tower oscillations are caused primarily by variations in the thrust force and torque acting on the rotor. Such variations are typically caused by turbulence in the wind or by changes in the controlling of the wind turbine, for example changing pitch or yawing. For the control of the wind turbine and with the general purpose of optimizing the power production and reducing the loads of the turbine it is important to know or have a good estimate of the wind speed and the thrust force acting on the rotor. Likewise, the position and velocity of the nacelle or the tower top are important parameters to be able to take into account in the wind turbine control. Both the positions and the velocities in the fore-aft direction and in the side-side direction relative to the orientation of the wind turbine are of importance.
The tower oscillation may at least to some extent be determined by placing a number of accelerometers and/or strain gauges along the entire length of the tower in order to thereby estimate the mode shapes of the oscillating tower. In another approach, the top position may be estimated based on an accelerometer positioned to measure the fore-aft accelerations of the nacelle together with an estimate on the thrust force acting on the rotor. However, this approach takes only the fore-aft motion into account and the use of a thrust estimate in the estimation unavoidably decreases the accuracy of the method.
US 2008/118342 discloses placing accelerometers or strain gauges in the tower head and approximately halfway up the tower of an turbine corresponding to where the maximum deflections of the tower occur under its first and second natural frequencies. If the measurements exceed certain limits indicative of excessive oscillations, the turbine is operated in trundle mode to dampen the oscillations.
US 2004/108729 discloses a wind turbine with means for detecting and monitoring the oscillation of the wind turbine pylon. Two one-way accelerometers are placed at right angles in the nacelle and the measured accelerations are cumulated and used in determining the average deflection of the pylon from the rest position. However, the oscillation is only determined by this average over time which is far from fully describing the oscillation or the travel of the nacelle during operation.
WO2010060772 discloses to control an operational parameter of a wind turbine blade. The velocity of the wind turbine nacelle and a position of the wind turbine nacelle relative to a running mean are determined and from these the actual operating situation. This actual operating situation is compared to a predetermined space of acceptable operating situations determined from a set of normal operating situations, and a control strategy is chosen from a predetermined set of strategies.